In an internal combustion engine, when, for example, ignition timing is controlled at MBT (Minimum Spark Advance for Best Torque), indicated mean effective pressure Pmi, which indicates combustion efficiency, becomes optimum. As shown in FIG. 20, the indicated mean effective pressure with respect to the ignition timing has an extremum characteristic.
Conventionally, such control is conducted by calculating the indicated mean effective pressure using a detected in-cylinder pressure and crank angle, averaging the result, determining the operating condition from the engine speed and load and, based on change of the indicated mean effective pressure when the ignition timing is made varied under the determined operating condition, controlling the ignition timing so that the ignition timing is to be the maximum value of the indicated mean effective pressure, as taught by, for instance, Patent Reference 1. The calculation of the indicated mean effective pressure is explained in Patent Reference 2 in detail.
However, the technique disclosed by Patent Reference 1 is not necessarily satisfactory in terms of control accuracy and response performance and, when a difference between predetermined ignition timing and optimum ignition timing occurs due to the manufacture variance or aging of the engine, the change of fuel property or the like, disadvantageously cannot compensate it.
Meanwhile, a theory called the Extremum-Seeking-Control is proposed as one of control theories, the details of which are described in, for example, Non-Patent Reference 1. The Extremum-Seeking-Control (hereinafter called “ESC”) is particularly efficient for searching the extremum in a plant having a non-linear characteristic. When the aforesaid ignition timing control is conducted, for instance, the use of this control theory can improve control accuracy and response performance and also enables to cope with the manufacture variance or aging of the plant, e.g., an internal combustion engine, the change of fuel property or the like. The similar extremum characteristic can be seen in a NOx emission characteristic of an engine, for example.
Patent Reference 1: Japanese Patent Publication No. 2536243
Patent Reference 2: Japanese Patent Publication No. 2695243
Non-Patent Reference 1: Real-Time Optimization by Extremum-Seeking Control (Kartic B. Ariyur and Miroslaw Krsitic: WILEY-INTERSCIENCE, 2003)
In the ESC, an input f for optimizing an output y is calculated by mixing a vibration input (perturbation input) p for detecting the relationship between output change and input change with an input u to be sent to a plant (controlled object), multiplying a change amount h of the output y at that time by the vibration input p and integrating the product.